748 research outputs found
Concentration of the first eigenfunction for a second order elliptic operator
We study the semi-classical limits of the first eigenfunction of a positive
second order operator on a compact Riemannian manifold when the diffusion
constant goes to zero. We assume that the first order term is given
by a vector field , whose recurrent components are either hyperbolic points
or cycles or two dimensional torii. The limits of the normalized eigenfunctions
concentrate on the recurrent sets of maximal dimension where the topological
pressure \cite{Kifer90} is attained. On the cycles and torii, the limit
measures are absolutely continuous with respect to the invariant probability
measure on these sets. We have determined these limit measures, using a blow-up
analysis.Comment: Note to appear in C.R.A.
Stellar model atmospheres with magnetic line blanketing
Model atmospheres of A and B stars are computed taking into account magnetic
line blanketing. These calculations are based on the new stellar model
atmosphere code LLModels which implements direct treatment of the opacities due
to the bound-bound transitions and ensures an accurate and detailed description
of the line absorption. The anomalous Zeeman effect was calculated for the
field strengths between 1 and 40 kG and a field vector perpendicular to the
line of sight. The model structure, high-resolution energy distribution,
photometric colors, metallic line spectra and the hydrogen Balmer line profiles
are computed for magnetic stars with different metallicities and are discussed
with respect to those of non-magnetic reference models. The magnetically
enhanced line blanketing changes the atmospheric structure and leads to a
redistribution of energy in the stellar spectrum. The most noticeable feature
in the optical region is the appearance of the 5200 A depression. However, this
effect is prominent only in cool A stars and disappears for higher effective
temperatures. The presence of a magnetic field produces opposite variation of
the flux distribution in the optical and UV region. A deficiency of the UV flux
is found for the whole range of considered effective temperatures, whereas the
``null wavelength'' where flux remains unchanged shifts towards the shorter
wavelengths for higher temperatures.Comment: accepted by Astronomy & Astrophysic
Stellar model atmospheres with magnetic line blanketing. III. The role of magnetic field inclination
Context. See abstract in the paper.
Aims. In the last paper of this series we study the effects of the magnetic
field, varying its strength and orientation, on the model atmosphere structure,
the energy distribution, photometric colors and the hydrogen Balmer line
profiles. We compare with the previous results for an isotropic case in order
to understand whether there is a clear relation between the value of the
magnetic field angle and model changes, and to study how important the
additional orientational information is. Also, we examine the probable
explanation of the visual flux depressions of the magnetic chemically peculiar
stars in the context of this work.
Methods. We calculated one more grid of the model atmospheres of magnetic A
and B stars for different effective temperatures (Teff=8000K, 11000K, 15000K),
magnetic field strengths (B=0, 5, 10, 40 kG) and various angles of the magnetic
field (Omega=0-90 degr) with respect to the atmosphere plane. We used the
LLmodels code which implements a direct method for line opacity calculation,
anomalous Zeeman splitting of spectral lines, and polarized radiation transfer.
Results. We have not found significant changes in model atmosphere structure,
photometric and spectroscopic observables or profiles of hydrogen Balmer lines
as we vary the magnetic field inclination angle Omega. The strength of the
magnetic field plays the main role in magnetic line blanketing. We show that
the magnetic field has a clear relation to the visual flux depressions of the
magnetic CP stars.
Conclusions. See abstract in the paper.Comment: 10 pages, 5 figure
Turbulent convection: comparing the moment equations to numerical simulations
The non-local hydrodynamic moment equations for compressible convection are
compared to numerical simulations. Convective and radiative flux typically
deviate less than 20% from the 3D simulations, while mean thermodynamic
quantities are accurate to at least 2% for the cases we have investigated. The
moment equations are solved in minutes rather than days on standard
workstations. We conclude that this convection model has the potential to
considerably improve the modelling of convection zones in stellar envelopes and
cores, in particular of A and F stars.Comment: 10 pages (6 pages of text including figure captions + 4 figures),
Latex 2e with AAS Latex 5.0 macros, accepted for publication in ApJ
Critical evaluation of magnetic field detections reported for pulsating B-type stars in the light of ESPaDOnS, Narval and reanalyzed FORS1/2 observations
Recent spectropolarimetric studies of 7 SPB and Cep stars have
suggested that photospheric magnetic fields are more common in B-type pulsators
than in the general population of B stars, suggesting a significant connection
between magnetic and pulsational phenomena. We present an analysis of new and
previously published spectropolarimetric observations of these stars. New
Stokes observations obtained with the high-resolution ESPaDOnS and Narval
instruments confirm the presence of a magnetic field in one of the stars
( Lup), but find no evidence of magnetism in 5 others. A re-analysis
of the published longitudinal field measurements obtained with the
low-resolution FORS1/2 spectropolarimeters finds that the measurements of all
stars show more scatter from zero than can be attributed to Gaussian noise,
suggesting the presence of a signal and/or systematic under-estimation of error
bars. Re-reduction and re-measurement of the FORS1/2 spectra from the ESO
archive demonstrates that small changes in reduction procedure lead to
substantial changes in the inferred longitudinal field, and substantially
reduces the number of field detections at the 3 level. Furthermore, we
find that the published periods are not unique solutions to the time series of
either the original or the revised FORS1/2 data. We conclude that the reported
field detections, proposed periods and field geometry models for Pyx,
15 CMa, 33 Eri and V1449 Aql are artefacts of the data analysis and reduction
procedures, and that magnetic fields at the reported strength are no more
common in SPB/ Cep stars than in the general population of B stars.Comment: 10 pages, 5 figures, accepted for publication in ApJ, 2012, typo
correcte
Electronic States in Diffused Quantum Wells
In the present study we calculate the energy values and the spatial
distributions of the bound electronic states in some diffused quantum wells.
The calculations are performed within the virtual crystal approximation, spin dependent empirical tight-binding model and the surface Green
function matching method. A good agreement is found between our results and
experimental data obtained for AlGaAs/GaAs quantum wells with thermally induced
changes in the profile at the interfaces. Our calculations show that for
diffusion lengths {\AA} the transition (C3-HH3) is not
sensitive to the diffusion length, but the transitions (C1-HH1), (C1-LH1),
(C2-HH2) and (C2-LH2) display large "blue shifts" as L_{D} increases. For
diffusion lengths {\AA} the transitions (C1-HH1) and (C1-LH1)
are less sensitive to the L_{D} changes than the (C3-HH3) transition. The
observed dependence is explained in terms of the bound states spatial
distributions.Comment: ReVTeX file, 7pp., no macros, 4 figures available on the reques
Investigating the post-stimulus undershoot of the BOLD signal â A simultaneous fMRI and fNIRS study
3
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Predictors of Stunting, Wasting and Underweight among Tanzanian Children Born to HIV-Infected Women.
Children born to human immunodeficiency virus (HIV)-infected women are susceptible to undernutrition, but modifiable risk factors and the time course of the development of undernutrition have not been well characterized. The objective of this study was to identify maternal, socioeconomic and child characteristics that are associated with stunting, wasting and underweight among Tanzanian children born to HIV-infected mothers, followed from 6 weeks of age for 24 months. Maternal and socioeconomic characteristics were recorded during pregnancy, data pertaining to the infant's birth were collected immediately after delivery, morbidity histories and anthropometric measurements were performed monthly. Multivariate Cox proportional hazards methods were used to assess the association between potential predictors and the time to first episode of stunting, wasting and underweight. A total of 2387 infants (54.0% male) were enrolled and followed for a median duration of 21.2 months. The respective prevalence of prematurity (<37 weeks) and low birth weight (<2500 g) was 15.2% and 7.0%; 11.3% of infants were HIV-positive at 6 weeks. Median time to first episode of stunting, wasting and underweight was 8.7, 7.2 and 7.0 months, respectively. Low maternal education, few household possessions, low infant birth weight, child HIV infection and male sex were all independent predictors of stunting, wasting and underweight. In addition, preterm infants were more likely to become wasted and underweight, whereas those with a low Apgar score at birth were more likely to become stunted. Interventions to improve maternal education and nutritional status, reduce mother-to-child transmission of HIV, and increase birth weight may lower the risk of undernutrition among children born to HIV-infected women
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